In the presence of proven structural risks, classic sounding methods offer no alternatives to closure to the public, as a precaution, despite the economic repercussions that entails. However, ongoing real-time tracking via optical cables makes it possible to accurately monitor the risk of spread of any problems and guarantee the continuity of operations.
Maintaining traffic flows during the bridge's replacement
Whether the Adour Bridge or the Francport Bridge, OSMOS has guaranteed the continuity of operation of multiple structures deemed to be in very poor condition, while a replacement bridge was being built. Thanks to the optical cables installed on the structures and the principle of continuous measurement, multiple parameters can be analyzed to provide managers with the information they need to maintain the flow of traffic until the bridge has been replaced, while ensuring user safety.
All structurally significant events, such as a general weakening of part of the structure caused, for example, by oxidation leading to irreversible microdeformation, are recorded and timestamped.
The principle of continuous measuring also enables the recording and timestamping of convoys passing over the bridge and causing significant deformation, the evaluation of the flexibility of structural elements like the deck and how it changes over time, and the verification of the elements' return to their original state after the trucks pass over them.
Finally, continuous measurement allows for analyses of modes of vibration when under
dynamic stress and of the structure's long-term behavior, making use of the average measurements over pre-defined timeframes (hour, day, week, month or year). Those analyses are then supplemented by a study of the correlations between variations in temperature and deformations, as well as monitoring of how those correlations change over time.
A new bridge under OSMOS's supervision
Since June 2016, OSMOS has been monitoring the structural condition of a bridge showing widespread oxidation, whose evolution requires close supervision.
Measuring 185 meters (607 ft) in length, this bridge has a metal framework with lateral girders in three independent, isostatic spans. Despite several attempts to reinforce its metal frame, it displays a state of widespread oxidation, particularly in the lower part of the deck, covering half of the pier side of the right bank span.
Leveling measurements taken between February and December 2015 revealed several instances of vertical displacement of around 6 mm in the middle of the span, on the upstream side and 2 mm in the middle of the span on the downstream side. In response to this finding, the structure's managers wanted to set up a monitoring system to more accurately quantify the displacement.
To be able to determine the changes in the deck's lateral girders at any time and detect any harmful phenomena, OSMOS proposed a continuous monitoring system that measures and records deformations in the lateral girders, with a high capture frequency (50 Hz to 100 Hz).
The system comprises a series of deformation sensors synchronized to a smart data capture terminal, allowing real-time data to be viewed remotely.
OSMOS's engineers determined that a series of optical cables could be installed in the middle of each span to accurately identify the general flexural behavior of the girders. As a result, each of the six girders was equipped with a longitudinal optical cable attached to the lower chord and two additional optical cables attached to the bottom chord of the two girders deemed to be the most critical.
Synchronization between the optical cables on the top and bottom chords accurately transmits the height of the girder's neutral axis and, consequently, makes it possible to identify any abnormal drift with greater certainty. Temperature sensors round out the system, one in the bottom chord and the other on the top chord of the middle span, quantifying both overall variations in temperature and the thermal gradient in the deck.
Mathematical tools for better knowledge of structures
Understanding the structure's behavioral based on its use or stresses, and extending its lifespan by adjusting its operating conditions: OSMOS offers its clients unique services, the fruit of advanced expertise in the field of structural monitoring, particularly for bridges.
Based on the observed deformations (due to tension and compression), as well as their magnitude, events' impacts on the structure can be tracked, their irreversibility can be studied, and weighing/counting modules can be put in place to improve knowledge related to the structures' operation.
Other, more complex mathematical methods are used to study the mechanical behavior of the structures. The coefficient of thermal expansion method aims, for example, to isolate the thermal component (linked to temperature variations) from the mechanical influence. A tendency can be derived from this (with a margin of error), representing the mechanical effects exercised on the structure. Conversely, another method consists of using the strong correlation between deformations and temperature variations to detect anomalies. In concrete terms, after a learning period that entails observing the structure's normal behavior, it is possible to use statistical data to check the structure's behavior and detect any anomalies.
For its part, the fatigue projection method consists of studying structural responses (fatigue) to a periodic number of events repeated over a very long period of time.
An analysis of statistical events enables a study, using all of the mathematical methods previously described, of the behavior of structures subjected to dynamic stress, as is the case of bridges. This method entails observing the intercorrelations between the different variables of dynamic events.